Many applications require the use of light steering mechanisms. Generally the light source that is being steered is a laser beam. Such applications include holographic data storage, line-scanned laser displays and bar-code scanners. Conventional methods of beam steering include galvanometric mirror scanners, rotating polygons, and phased array liquid crystal devices. Galvanometric scanners and rotating polygons are based on moving elements, and suffer from long-term reliability issues due to wear and tear. Rotating a polygon also consumes a significant amount of power and the size and form factor of a rotating polygon may present additional disadvantages when integrated into a device.
Liquid crystal phased arrays have been used that do not contain any moving elements, but that contain a very large number of address electrodes. For example, a primary problem in using a conventional stair-step piston type nematic liquid crystal phased array for beam steering is the requirement of using a large number of electrodes. Each electrode may have a separate voltage applied to it in order to achieve the desired optical steering characteristic. Using this technique, it is common to have a “stair step” voltage from electrode to electrode. That is, the applied voltage is raised by a certain level between successive adjacent electrodes in one dimension. For example, the voltage could be 1 volt at a first electrode, 2 volts at a second electrode, 3 volts at a third electrode, and 4 volts at a fourth electrode. Then repeat to start a new stair step function, with the voltage again being 1 volt at a fifth electrode, 2 volts at a sixth electrode, 3 volts at a seventh electrode, and 4 volt at an eighth electrode. In this example, the voltage values repeat after every four electrodes. This set of four electrodes constitutes a step-wise voltage ramp which is an approximation for a linear voltage ramp.
There are multiple sets of these ramps in a phased array device. The voltage ramp corresponds to a phase ramp in the liquid crystal device across which these multiple voltages are applied. In order to achieve high throughput diffraction efficiency, the phase ramp has to have a large number of voltage steps, which means high pixel count and complicated electronic drive schemes for generating a large number of voltage levels. Also, multiple small electrodes have to be cramped in a small space for the large number of pixels or electrodes in the voltage ramp. The larger the number of voltage steps, the better the linear voltage ramp approximation. Because adjacent pixels are at different voltage levels, there needs to be electrical isolation between these pixels in order to prevent electrical short and obtain the desired voltage ramp. This electrical isolation is in general achieved with gaps between adjacent electrodes. This gap results in lesser amount of light coupling into the desired diffraction order, thereby reducing diffraction efficiency. Also, in order to achieve large steering angles, there need to be multiple sets of electrodes for the voltage ramp in a very short space. All of these requirements makes the phased array difficult to realize. The pixilation of electrodes results in a large number of electrodes to control, lower throughput and lower diffraction efficiency, smaller steering angles, large array size, and complex driving circuitry. This also makes the fabrication and testing of the phased array very difficult and cumbersome.
Thus, it would be advantageous to have a phased array approach which addresses the foregoing issues, which is non-pixelated, which provides high throughput and high diffraction efficiency, and which enables large steering angles.